1. Field of the Invention
The present invention relates to an air fuel ratio control apparatus for an internal combustion engine installed on a vehicle or the like. In particular, the invention relates to an air fuel ratio control apparatus for an internal combustion engine provided with an air fuel ratio feedback control section for oscillating the air fuel ratio of a mixture supplied to the internal combustion engine in rich and lean directions in a periodic manner.
2. Description of the Related Art
In general, a three-way catalyst (hereinafter referred to simply as a “catalyst”) for purifying harmful components HC, CO, NOx in an exhaust gas at the same time is installed in the exhaust passage of an internal combustion engine, and in this kind of catalyst, the purification rate of the harmful components HC, CO, NOx becomes high in the vicinity of the stoichiometric air fuel ratio. Accordingly, in air fuel ratio control apparatuses for an internal combustion engine, an oxygen sensor is generally arranged at a location upstream of the catalyst, and the air fuel ratio of a mixture is controlled in a feedback manner by adjusting the amount of injection fuel so as to control the air fuel ratio to a value in the vicinity of the stoichiometric air fuel ratio.
In addition, an oxygen occlusion capability, acting like filter processing, is added to the catalyst, so that a temporary variation of an upstream air fuel ratio (corresponding to an output value of an upstream oxygen sensor) from the stoichiometric air fuel ratio is absorbed. That is, the catalyst takes in the oxygen contained in the exhaust gas when the upstream air fuel ratio (hereinafter referred to as an “upstream A/F”) is leaner than the stoichiometric air fuel ratio, whereas it releases the oxygen accumulated in the catalyst when the upstream A/F is richer than the stoichiometric air fuel ratio. Accordingly, the variation of the upstream A/F is filter processed in the catalyst, thus resulting in an air fuel ratio downstream of the catalyst.
Also, a maximum value of the amount of oxygen occlusion of the catalyst is decided by an amount of a material having an oxygen occlusion capability attached upon production of the catalyst, and the variation of the upstream A/F can not be absorbed any more when the amount of oxygen occlusion reaches a maximum amount of oxygen occlusion or a minimum amount of oxygen occlusion (=0) of the catalyst, so the air fuel ratio in the catalyst deviates from the stoichiometric air fuel ratio to decrease the purification ability of the catalyst. At this time, the air fuel ratio downstream of the catalyst deviates greatly from the stoichiometric air fuel ratio, so it is possible to detect that the amount of oxygen occlusion in the catalyst has reached the maximum value or minimum value (=0).
Further, the catalyst, being exposed to the exhaust gas of a high temperature, is designed such that the purification function of the catalyst is not rapidly reduced in use conditions which can be generally considered in the internal combustion engine for a vehicle. However, the oxygen occlusion capability of the catalyst might remarkably be decreased during the use thereof because of some causes (e.g., in case of a misfire). In addition, the oxygen occlusion capability is decreased gradually due to aging even under an ordinary condition of use when the travel distance of the vehicle reaches tens of thousands of kilometers for example.
On the other hand, in recent years, there has been proposed an air fuel ratio control apparatus for an internal combustion engine in which by focusing attention on the fact that when the amount of oxygen occlusion of a catalyst is oscillated a predetermined quantity within the range of a maximum amount of oxygen occlusion, the purification ability of the catalyst is improved, the width (amplitude) of oscillation of the amount of oxygen occlusion is changed adaptively with respect to the change of the maximum amount of oxygen occlusion of the catalyst due to the degradation of the catalyst or the temperature of the catalyst, so that the purification ability of the catalyst is drawn out to its maximum regardless of the degradation thereof (see, for example, a first patent document: Japanese patent application laid-open No. H 7-259600).
In addition, there has also been proposed a further air fuel ratio control apparatus for an internal combustion engine in which by focusing attention to the principle that the variation of a downstream air fuel ratio (hereinafter referred to as a downstream “A/F”) of a catalyst becomes large when the width of oscillation of the amount of oxygen occlusion has gone off (deviated from) a maximum amount of oxygen occlusion of the catalyst, the degradation of the catalyst is diagnosed from a quantity of variation of the amount of oxygen occlusion when the variation of the downstream A/F is increased by changing the width of oscillation of the amount of oxygen occlusion (see, for example, a second patent document: Japanese patent application laid-open No. H6-26330).
In the conventional apparatus described in the above-mentioned first patent document, in order to change the width of oscillation of the amount of oxygen occlusion, the period and oscillation width (amplitude) of the air fuel ratio oscillation to rich and lean directions of the upstream A/F is caused to change, as shown in timing charts of FIG. 34, FIG. 35.
That is, in case of a normal catalyst, a maximum amount of oxygen occlusion OSCmax is large, as shown in the timing chart of FIG. 34, so it is possible to set the width (amplitude) ΔOSC of oscillation of the estimated amount of oxygen occlusion OSC (hereinafter simply referred to as an “amount of oxygen occlusion”) to a large value within the range of the maximum amount of oxygen occlusion OSCmax, and the oscillation width or the period of the variation of the upstream A/F can be made large thereby to be able to set the width of oscillation ΔOSC of the amount of oxygen occlusion to a large value.
On the other hand, in case of a degraded catalyst, the maximum amount of oxygen occlusion OSCmax is small, as shown in the timing chart of FIG. 35, so the width of oscillation ΔOSC of the amount of oxygen occlusion is set small within the range of the maximum amount of oxygen occlusion OSCmax, and the oscillation width or the period of the variation of the upstream A/F can be made small thereby to set the width of oscillation ΔOSC of the amount of oxygen occlusion to a small value.
As stated above, in the conventional air fuel ratio control apparatus for an internal combustion engine described in the above-mentioned first patent document, it is necessary to greatly change the oscillation width or period of the air fuel ratio oscillation (see FIG. 34 and FIG. 35) in accordance with the change of the maximum amount of oxygen occlusion OSCmax.
In the conventional air fuel ratio control apparatuses for an internal combustion engine, it is necessary to change the oscillation width or the period of the air fuel ratio oscillation in accordance with the change of the maximum amount of oxygen occlusion, as can be seen in the first patent document for example, as a result of which a large influence is given to the air fuel ratio feedback performance and the torque variation. so there is a problem that controllability of the air fuel ratio is deteriorated.
In addition, there is another problem that when an external disturbance occurs in case where the oscillation width or the period of the air fuel ratio oscillation becomes large, the performance to make the air fuel ratio oscillation converge into a steady state is deteriorated, thus reducing the exhaust gas (emission) performance upon acceleration or deceleration.
Moreover, torque variation is caused by a change in the air fuel ratio, so when the oscillation width or period greatly changes, driveability of the vehicle is deteriorated to reduce the marketability thereof, as a result of which there is a problem that it is difficult to set a setting condition for the oscillation processing of the amount of oxygen occlusion, a setting condition for placing greater importance on the feedback performance, and a setting condition for placing greater importance on the torque variation, separately from one another.
Further, in order to cope with the exhaust emission control which is specified in a variety of manners all over the world, it is necessary to change catalysts in accordance with regulations of individual countries and places so as to change the maximum amount of oxygen occlusion in a variety of ways. Therefore, there has been a problem that it is necessary to set the width or period of the air fuel ratio oscillation for each catalyst, so the adaptation or compatibility costs become large. Further, there are also a variety of exhaust emission regulations for catalyst degradation diagnosis, so there has been a problem that it is necessary to adapt the width or period of the air fuel ratio oscillation so as to meet regulations of individual countries and areas.
In addition, in recent years, exhaust emission control is strengthened from enhanced consideration to the earth environment, and hence it is requested to set the period or width of oscillation of an air fuel ratio to a large value so as to detect much smaller degradation of a catalyst (a decrease in the maximum amount of oxygen occlusion). As a result, there has been a problem that there is a tendency to invite various kinds of performance deteriorations such as a deterioration in air fuel ratio feedback performance, an increase in torque variation, etc.
Further, in recent years, the thermal resistance of materials having an oxygen occlusion capability has been improved year by year, and the amount of addition of such materials to catalysts has been able to be increased. Accordingly, a maximum amount of oxygen occlusion is increasing, so it is required to set the period or width of the oscillation of an air fuel ratio as greatly as possible, as a consequence of which there has also been the problem of tending to invite various deteriorations of performance such as a deterioration in air fuel ratio feedback performance, an increase in torque variation, etc.